SSP421 Body Basics

Service Training

Self-study Programme 421

Body Basics

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A modern body needs to retain a good condition, if possible, for the whole life of the car so that no unnecessary costs arise. This prevents a fast moral decline of a vehicle model and the brand will be held in high esteem.

It is also important that damage to a body that occurs in any future accidents can be repaired and this high-quality vehicle component can thus retain its properties and appearance.

This self-study programme will explain the basic associations of material science, steel production and the treatment processes leading up to painting. It will lay the foundations for better understanding of the procedures should body repairs be necessary.

The self-study programme portrays the design and function of new developments. The contents will not be updated.

For current testing, adjustment and repair instructions, refer to the relevant service literature. Important

Note

What springs to mind when you think about a vehicle? Normally the drive, the engine, the performance and the image come first.

But what would a car be without its body? It is the central component of a vehicle that connects all other vehicle components and, at the same time, manages to accommodate the passengers.

It meets high technical requirements as well as the comfort expectations of passengers.

Furthermore the body in particular bears the characteristic face of a vehicle and thus also of the car brand.

3Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Historical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Basics - Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Basics - General information on materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Basics - Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Basics - Treatment of steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Basics - Aluminium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Steels for Body Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Classification of steels for body construction . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Increasing the strength of steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Body structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Basics - Process Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40Manufacture of semi-finished products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Tailored blanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Joining processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Coating processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Corrosion Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Pre-treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Seam sealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Stone chip protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Protection against galvanic corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Painting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Repair methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 The glossary provides explanations of the CAPITALISED terms

Contents

4Introduction

Historical background

Motor carriage Wood/steel

Separate body on drivable chassis, e.g. Ford Model T

Non-load-bearing body

The first bodies were not load-bearing, i.e. they were built on a frame or a chassis. Both ladder frames and tubular frames were used. Ladder frames are still used for lorries and off-road vehicles today. Bodies were not necessarily linked to one vehicle manufacturer. There were chassis manufacturers and coachbuilders. This allowed bodies to be adapted to and built onto different drivable chassis as required. Later on bodies were produced using a platform frame. The body structure was placed on a platform frame that had been specially designed for it and was then attached. Only then was it fitted onto the drivable chassis.

Introduction

Car bodies yesterday and today - how have requirements and tasks of car bodies changed over the course of technical progress? To begin with, let us define the term body, which refers to the complete coachwork of a motor vehicle. Historically, the first superstructure on a vehicle with wheels could be considered as the start of body development. We should then also include the first open and later closed motor carriages.

We will describe bodies in the context of motor vehicles throughout this self-study programme.

from the start of motor vehicle production to this day

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Body with platform frame Current monocoque body

Monocoque bodies

Engineers looked for solutions to make bodies without a separate, heavy frame as it became necessary to reduce weight in car construction. The resulting body featured a structure that could support itself. This development was also aided by technical progress in sheet metal processing. In this design, the load-bearing body structures are joined together securely and permanently using different joining techniques, for example, welding, soldering and bonding. This method of construction is the general standard in car manufacturing today.

Use of materials

The first bodies were made of wood, to which an outer coating of lacquer, fabric or plastic was added to improve the appearance and to also protect it from the elements. Modern bodies are increasingly not just optimised in terms of structure and cross-sections. Special tailor-made materials are being used more and more for specific body areas. The materials are distinguished by their respective composition and their treatment. These different materials are used to continually improve the ability of specific sections of the body to bear mechanical stress and resist corrosion. Above all, steel is used as a material. Aluminium and plastics are also being used more to create lightweight constructions.

6Requirements for bodies

As a result of technical progress, todays bodies have an increasingly complex range of tasks to perform:

Transport space for passengers

Fulfilment of human needs, for example, comfort, noise insulation etc.

Accommodation of all technical components for drive and power transmission

Accommodation of all technical components for convenience systems

Accommodation of heating, ventilation and air-conditioning systems

Accommodation of all safety systems

Design of the body for optimum protection against possible interior and exterior damage, e.g. in accidents

Image aspects

Structure of a body

The body can be split into two main areas:

the inner body structure and the outer panels of the body structure

Due to safety requirements, the inner part of the body structure, also called the passenger cell, needs to be particularly resistant to deformation.

The front and rear ends of the body should allow optimum absorption and reduction of impact energy (crumple zones to meet safety requirements in a crash). This also applies increasingly for the side sections and the roofs of modern vehicles.

Repairs to a body

Repairs to the inner body structure are very complex due to its sturdy configuration.

Parts fastened to the body structure with bolts (body outer skin) are replaced if badly damaged.

Introduction

BodiesBodies are frequently only perceived and understood as a design. People normally only consider the technical issues and comfort as an afterthought.

In a nutshell, the body is first and foremost the part of a vehicle that allows passengers to be transported - it is the actual transport case for the people being carried.

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The structure of a body will be shown using the bodies of the current Golf and Passat as examples.

The body shows the internal body structure together with parts of the outer panels.

With outer panels

Body structure,outer panels transparent

Body of Golf model year 2009

Body structure,outer panels transparent

With outer panels

Body of Passat model year 2006

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Basics - Materials

Crystalline structures

All substances are made from different types of ATOMS (iron, sodium, gold, nitrogen, oxygen etc.) regardless of their state of matter (gas, liquid or solid). The ATOMS behave like solid balls, whose size varies depending on their nature.

The following descriptions cover the relationships with the crystalline structures using metals as an example. When the metals are still in liquid state during melting, the ATOMS move randomly without staying in a set position.

The crystal lattice shown is a random section of a crystalline structure. The ATOMS highlighted in yellow in the illustration show an example of a basic unit of the crystalline structure - in this case a simple cubic crystal. The possible crystalline forms are explained in more detail on the next page.

Basic unit (simple cubic)

Crystalline structure

Basics - General information on materialsOn modern bodies, specially configured materials are increasingly being used for specific body parts. In order to explain the reasons for the respective materials used, we will look at some principles of material science.

When the materials become solid upon cooling, the ATOMS stop moving and arrange themselves in a set three-dimensional form, i.e. crystalline structure.

The crystalline structure can be reduced to the respective smallest basic unit. In its simplest form, the basic unit can be, for example, simple cubic.

Depending on the type of material, body-centred cubic and face-centred cubic or even hexagonal crystalline forms, for example, are possible in addition to the simple cubic forms.

The crystalline structure type determines characteristic material properties like density, HARDNESS or melting point.

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Metals mostly crystallise in three crystalline forms:

Body-centred cubic crystal The METAL IONS arrange themselves so that the connecting lines between the ions form a cube. Another metal ion is located in the centre of the cube. This crystallisation type is found in chrome, molybdenum, vanadium and tungsten as well as in iron below approx. 900C.

METAL IONS = electrically charged atoms

Face-centred cubic crystal The basic form of the crystal is also a cube. In addition to the eight METAL IONS at the corners, there is one ion in the centre of the six side surfaces. This crystallisation type is found, for example, in lead, aluminium, copper and nickel as well as in iron above approx. 900C.

Hexagonal crystal The basic form of this crystal is a prism with hexagonal top and bottom surfaces. There is also a METAL ION in the centre of the top and bottom surfaces. In addition, three further METAL IONS are arranged inside the crystal. This crystallisation type is found, for example, in magnesium, titanium and zinc.

Face-centredcubic

Body-centredcubic

Hexagonal

METAL IONS

METAL IONS

METAL IONS

10

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Basics - Materials

Metal structure

In liquid molten metal, METAL IONS and free ELECTRONS move randomly among each other. There is no specific arrangement.

An arrangement only forms once solidification begins. Formation of crystallisation seeds begins at the points where the solidification temperature is reached first.

Start of solidificationLiquid molten metal

Iron is not used in its pure form for technical applications. Instead it is used in a wide range of ALLOYS. Iron and alloy components are melted together. The alloy additions dissolve in the iron base metal during this process. Different structure types are formed depending on the mixing ratio of iron and alloy components.

As solidification continues, METAL IONS are deposited on these seeds and crystal lattices form. The crystal lattices continue to grow until they meet other neighbouring crystal lattices that are also growing. They cannot grow any further.

The crystals formed after complete solidification are irregular in their exterior form - they are also called crystallite or grains - they form the structure of a metal.

Continuing solidification Complete solidification

Crystallisation seeds

Growing crystals Grains

METAL IONS

Grain boundaries

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Structure types

Solid solution alloy

When ATOMS from the base metal are replaced by an ion from the alloy metal in a crystal, this is known as a substitutional SOLID SOLUTION.

If the ions of the alloy metal arrange themselves between the METAL IONS, this is known as an interstitial SOLID SOLUTION.

Multi-phase alloy

If alloy components do not mix upon solidification of the melt and each component forms its own crystals, this is known as a multi-phase alloy.

Plain steel has one special feature. The alloy component in this case is carbon. It is chemically bonded in the steel as iron carbide (cementite) and forms thin layers between the grains of the iron crystals (ferrite) - these grains are called pearlite.

Structure with substitutional solid solution crystals

Grain Grain boundary

Structure with interstitial solid solution crystals

Structure with multiple phases

Grain Grain boundary

Base metal Alloy metal

Structure of plain steel Cementite

Ferrite grain Pearlite grain Ferrite

12

ReH

ReL

Rm Z

S421_008 S421_093

Rp 0,2

X

Basics - Materials

Mechanical properties

The mechanical properties of materials are established in laboratory tests. Tensile testing for determining the strength of materials is the most common.

In tensile testing, samples of the test material are subjected to a rising pulling force at room temperature. All samples must have a standard size and shape so that the results for different materials can be compared.

The following two characteristic curves are examples - they can differ from each other considerably depending on the material or its variation.

During the test, the load required to deform the sample until it breaks and the extent of deformation is determined. A stress-strain curve (load in relation to the original cross-section of the sample) can then be drawn up from the results.

Depending on the type of material, an extended yield point (yield-point runout) or a continuous transition from the elastic region to the plastic region (with 0.2% offset yield point) can occur on the curve.

Stress-strain curve- with extended yield point -

Stre

ss (M

Pa)

Stress-strain curve- with 0.2% offset yield point -

Stre

ss (M

Pa)

The normal unit from the international measuring system, i.e. pascal (Pa), is used. Due to the very high values, megapascals (MPa) are mainly used: 1MPa = 1,000,000Pa. The measuring unit newton (N) per square millimetre is still also used, however: 1MPa = 1N/mm2.

Strain in % Strain in %

Plastic region

Elastic region

13

Yield point ReH and ReL

At the start of load application, the strain on the sample is elastic, i.e. the sample rod returns to its original length once the load is removed. This elastic behaviour continues until the yield point ReH is reached. Therefore this point can also be called the elastic limit. (This is an example and a simplified approach for this self-study programme. In practice, there are, of course, numerous variants of the curve where, for example, the elastic limit can be just in front of the yield point. These cases will, however, not be examined further in this self-study programme). The elastic limit separates the elastic region from the plastic region.

The sample starts to deform plastically once the yield point ReH (elastic limit) has been passed. Depending on the type of material, the yield point can also cover a region (the yield-point runout X) with an upper yield point ReH and a lower yield point ReL (see figure S421_008).

If there is no extended yield point, the 0.2% offset yield point is set as a substitute yield point. This is the point where the 0.2% remaining strain is found. A line is drawn parallel to the linear rise at a distance of 0.2% strain for this purpose. The point where this line crosses the curve is the offset yield point Rp 0.2 (see figure S421_093).

Ultimate strength or tensile strength Rm

The stress reaches maximum here. The stress Rm is the highest load that a cross-section can withstand. From this point on, the sample starts to neck, i.e. the cross-section area becomes smaller. Since the stress is calculated as a force in relation to the cross-section area, the force required for further deformation decreases after point Rm.

Fracture point (Z):

The sample fails at this point. This value has no significance for technical practice.

Analysis of the stress-strain curve

A detailed analysis of the stress-strain curve provides a range of information among which the following should be highlighted:

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Ba

y cropd tposics - Materials

Basics - SteelSteel is one of the most common materials in vehicle bodwith a maximum carbon proportion of 2.06%. Greater pmanganese, phosphor, silicon, chrome etc. can be alloyenumber of steel types, each of which has a different com

Production of steelFuIron ore extraction

The iron ore found in the ground is smelted in a furnace to form iron (pig iron). Depending on the composition of the iron ore used, the smelted pig iron has its own specific composition.

The resulting pig iron is then processed at the steelworks to adjust its chemical composition to previously specified and required values. This can be achieved by removing and adding certain elements. The process is also called alloying.

F

Ore chargingonstruction. Steel is an ALLOY of iron and carbon ortions result in castings. Further elements like nickel,

o produce the different steel families. There are a large sition and thus different properties.S421_070

Steelworksrnace

Different types of steel can be produced by varying the carbon content and other alloying elements. Setting the carbon content up to around 2.06% in particular makes a steel from the iron. The specific properties of the steel can be specially configured depending on the later use of the material.

Foundry ladle

Distributing trough

SlabIngot

mould

Billet discharge

urnace

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Chemical composition

Each type of steel has its own crystalline structure in solid state. This structure gives it mechanical properties that can be determined using a metallographic analysis.

One very important aspect of steel production is the speed at which the raw materials are heated and cooled in both the manufacturing process and subsequent procedures. If you examine iron in its pure state, you can determine how its crystalline structure and its magnetic properties and its solubility vary as the temperature rises or falls.

To give a better understanding of the properties of a type of steel, first the properties of pure iron, then the properties of different iron-carbon ALLOYS and finally the properties of ALLOYS made from iron, carbon and other components will be explained.

At the steelworks, the melt is conveyed to the continuous casting plant and processed into a billet. This billet material then forms the basic product for all further processes, for example, rolling, thermal treatment and surface coating, to produce a wide range of SEMI-FINISHED PRODUCTS.

The steel properties are determined by the following main variables:

the chemical composition of the steel the later treatment of the steel products

Pure iron

To provide a basis for all further observations of the steel types, the temperatures at which pure iron changes its crystalline structure, i.e. the arrangement of its ATOMS need to be known.

This new arrangement requires a certain amount of time during which the temperature remains constant. The temperatures where the structure changes are called critical points (see diagram Transformation points of pure iron on the next page). These critical points are shown on the diagram as horizontal lines resembling steps.

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e

d

c

b

a

Basics - Materials

Transformation points of pure iron

Tem

pera

ture

(C

)

Time (min.)

At the critical transformation points, the absorbed heat is used to change the crystalline structure. Therefore there is no rise in temperature. This means that the iron atoms group themselves in different crystalline structures with different magnetic properties and different carbon solubility depending on the temperature.

Heating

At temperatures between 0 and 790C, iron is called alpha iron or ferrite and has a body-centred cubic crystalline structure consisting of nine iron atoms. Alpha iron is very magnetic and does not dissolve carbon.

If the iron is heated further to 790 to 910C, it is called beta iron. It still has a body-centred cubic crystalline structure that also does not dissolve carbon, but it loses part of its magnetism.

Between 910 and 1,400C, it is called gamma iron. The cubic structure becomes face-centred at this stage. Gamma iron is not magnetic and dissolves carbon.

Between 1,400 and 1,535C, it is called delta iron. The crystalline structure is body-centred cubic. Since it only occurs at very high temperatures, it is only of little importance in the investigation of thermal treatment types.

At temperatures above 1,535C, pure iron is liquid.

In comparison with heating, the opposite process occurs during cooling (with minor deviations in the critical points).

Liquid iron

Delta iron

Gamma iron

Beta iron

Alpha iron or ferrite

Cooling curve

Heating curve

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Transformation points of iron-carbon alloys

Steel crystallises in different microstructures depending on the temperature and carbon proportion. Steel is an iron-carbon alloy with a maximum carbon proportion of 2.06%.

Interpretation of iron-carbon diagram

The lines on the diagram show the critical points, at which the steel changes its crystalline structure. Depending on the carbon proportion, the temperature at which the structural change occurs varies. The areas bordering on the critical points show the crystalline structure that the steel adopts in each case. At high temperatures, the steel is molten and all of its components are dissolved - like salt in water.

As the steel cools, parts of the iron and the carbon solidify. The so-called austenite crystals form (the solidification temperature varies depending on the carbon proportion). If the temperature decreases further, the ALLOY solidifies completely to become austenite.

Under approximately 723C, austenite can form two different structures depending on the carbon proportion in the ALLOY:

If the carbon proportion is below 0.8%, austenite becomes pearlite and ferrite, which are distributed within the steel in crystal form.

If the carbon proportion is 0.8%, the steel will only contain combined pearlite crystals. If the carbon proportion is greater than 0.8%, austenite becomes pearlite and cementite.

Austenite + melt

Pearlite + cementite

Carbon in %

Austenite

FerritePearlite + ferrite

Tem

pera

ture

in

C

Iron-carbon diagram

Austenite + cementite

Ferrite + austenite

Melt The illustration shows only the area of the iron-carbon diagram that is relevant to steels. The complete version of the diagram reaches to over 6% carbon.

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Type of microstructure

Explanation

Ferrite Ferrite consists of alpha-iron crystals, which form a body-centred cubic crystal lattice.Carbon is deposited between the iron atoms (interstitial SOLID SOLUTION).

Ferrite is the softest component of steel.

In terms of mechanical properties, ferrite is characterised by an ultimate strength of 28MPa and a strain of 35 to 40%.

Carbon Pure carbon atoms can be connected to each other in two ways. Graphite or diamond results depending on the type of bonding.

Of course, for steels, only carbon in the form of graphite is important. Its bonds are weak and therefore easily broken.

Carbon is either incorporated atomically in the lattice (interstitial SOLID SOLUTION) or in the form of bonds - see, for example, cementite.

Cementite Cementite is a chemical compound of iron and carbon. In chemistry, it is known as iron carbide (Fe3C).Cementite contains 6.67% carbon and 93.33% iron.

Cementite can be deposited in the structure in a different form, in separate formations (e.g. rods or needles) or in groups on the grain boundaries of the crystalline structure.

Cementite is the hardest, but also most brittle component of steel with an ultimate strength of 215MPa.

Austenite Austenite consists of gamma-iron crystals, which form a face-centred cubic crystal lattice.

It contains carbon, which is deposited between the iron atoms like with ferrite (interstitial SOLID SOLUTION). Due to the larger interstices in the face-centred cubic crystal lattice, more carbon can be incorporated compared with ferrite.

The carbon proportion varies between 0 and 2.06%.

Austenite has an ultimate strength of 88 to 105MPa and a strain of 20 to 23%. It is characterised by a high wear resistance and low HARDNESS and is the substance that occurs most frequently in steels.

Basics - Materials

Structure types

The table below explains the most important structural and mechanical properties of the compositions listed in the iron-carbon diagram (only the area of the diagram relevant to steel). (The listed compositions also occur above this area i.e. with > 2.06% carbon.)

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Type of microstructure

Explanation

Pearlite Pearlite is a mixture of cementite and ferrite layers that results from austenite transformation. The thickness of the layers depends on the cooling speed. The slower the cooling, the thicker the layers.

Pearlite has an ultimate strength of 5570MPa and a strain of 2028%.

The mechanical parameters of pearlite are between those of ferrite and cementite; it is harder and more resistant than ferrite, but softer and easier to shape than cementite.

Martensite Martensite is produced by cooling austenite very quickly.

During this process, the face-centred cubic lattice of the austenite is transformed into a body-centred cubic lattice. More carbon is dissolved in the face-centred cubic lattice due to there being more space available than in the body-centred cubic lattice. As a result, the carbon cannot find enough space during the structural transformation, distorts the crystal lattice and expands it. Plate-like crystals are formed, which look like needles in the microsection - so-called martensite needles.The compression stress caused by the greater volume of the martensite results in the greater HARDNESS, but also the brittleness of the martensite.

As the cooling speed increases, less and less pearlite is formed so that an almost complete transformation into martensite occurs.

After cementite, martensite is one of the hardest components of steel. The ultimate strength of martensite is between 170 and 250MPa and the strain is between 0.5 and 2.5%.

Bainite Like pearlite, bainite consists of the ferrite and cementite phases, but differs from pearlite in form, size and distribution.

Bainite has a structure containing either bundles of ferrite laths with film-like carbide layers between them or ferrite plates with layers of carbide between them.

Bainite is formed by accordingly controlled fast cooling of the austenite at temperatures and cooling speeds between those for pearlite and martensite. After cooling the austenite to temperatures above the martensite start temperature, the conversion into bainite occurs in the so-called bainite phase at a now constantly maintained temperature (isothermal).

The cooling speed is controlled so that no pearlite is formed.

The ultimate strength ranges between 15 and 220MPa and the strain is 1.5 2%.

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Basics - Materials

In the following section, two examples of structural changes, which occur in the production of steel with a carbon proportion of 0.15 or 1.20%, are explained by way of illustration.

The starting temperature is over 1,500C - the steel is liquid.

If the temperature falls below 1,400C, part of the iron and the carbon solidifies and a mixture of a liquid phase and austenite is created.

When the temperature drops below 1,300C, the steel with 0.15% carbon proportion is already completely solid and has become austenite. It remains like this until the temperature falls below 750C.

Between 750 and 723C, a mixture of austenite and ferrite is formed.

Below 723C, the crystalline structure changes and a mixture of pearlite and ferrite is formed. If, during this phase, the cooling speed changes at any point, differing structure components, like, bainite, martensite etc., can form.

Ferrite

Austenite

Pearlite

Liquid phase

Austenite

Austenite

Ferrite

Steel with 0.15% carbon

21

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This steel type is also liquid at 1,500C.

Between 1,300 and 1,200C, a mixture of a liquid phase and austenite occurs.

When the temperature drops below 1,200C, the steel with 1.20% carbon is completely solid and has become austenite.

Between 1,000 and 723C, a mixture of austenite and cementite is formed.

At temperatures below 723C, the crystalline structure is transformed into a stable state and forms a mixture of pearlite and cementite.

Austenite

Liquid phase

Cementite

Cementite

Austenite

Austenite

Pearlite

Steel with 1.20% carbon

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Basics - Materials

Alloying steel with further components

The carbon proportion is responsible for the STRENGTH of the steel.

If selected further alloy elements, for example, silicon, phosphor, titanium, niobium or chrome are added, other specific properties can be created as required.

The effects of these elements are even noticeable if they are added only in small quantities in relation to the other alloy components.

The ALLOYING of an iron-crystalline structure can happen in the following different forms:

Substitution: ATOMS of other elements replace iron atoms in the crystal matrix.

Gap: Iron atoms are removed from the crystal matrix and the corresponding space remains free.

Interstitial: ATOMS from other elements like titanium occupy interstitial* positions between the iron atoms of the crystal matrix. They hinder possible sliding of the atom surfaces against each other and thus increase the strength, for example. Their quantity in the steel alloy is decisive for the later mechanical properties.

Crystalline structure/crystal matrix

* Interstitial - located in an intermediate position within a structure

ATOM of base material

Substitution

Gap

Interstitial

23

Alloying element

Changed properties of the steel

Chrome Increases the passivity of the steel towards corrosive influences (it is the main alloying element for increasing the rust- and acid-resistance of steels)

Manganese Refines the grain; increases the STRENGTH; improves the temperability; increases HARDNESS, strain and wear resistance; improves weldability and malleability

Molybdenum Increases the STRENGTH and the tenacity; enhances the passivity of the steel towards corrosive influences; improves the temperability, reduces the tempering brittleness in CrNi and Mn steels; favours fine grain formation and improves the weldability.

Nickel Increases the STRENGTH and tenacity; contributes to stabilisation of the austenitic lattice structure; improves the formability even at low temperatures

Niobium Niobium behaves similar to titanium

Phosphor Increases the STRENGTH; contributes to an equal balance between press-formability and mechanical STRENGTH

Silicon Increases the STRENGTH and the elastic limit; refines the grain

Nitrogen Increases the STRENGTH of austenitic steels; improves the mechanical properties at higher temperatures

Titanium Increases the STRENGTH and tenacity; inhibits grain growth and thus contributes to a fine-crystal microstructure; suppresses the precipitation of chrome carbide in chrome alloy steels and thus grain boundary corrosion

The table shows you how the steel properties can be influenced with examples of some further important alloy elements.

24

Process Procedure

Cold forming The steel is formed below its RECRYSTALLISATION temperature. The HARDNESS, the deformation resistance and the elasticity limit of the steel are increased.

Hot forming The steel is formed above its RECRYSTALLISATION temperature. In this treatment, plastic deformation and the RECRYSTALLISATION of the deformed grains occur simultaneously whereby the temperature has to be maintained long enough so that complete RECRYSTALLISATION occurs. This treatment allows the same forming as in cold forming, but with less force requirement resulting in softer, more malleable steels with a more standardised crystalline structure.

Treatment type Treatment

Mechanical treatments Cold forming

Hot forming

Thermal treatments Hardening

Annealing

Normalising

Tempering

Thermochemical treatments Case hardening

Carbonitriding

Nitrating

Basics - Materials

There are numerous types of treatment that can be grouped into three large families:

Basics - Treatment of steel

Mechanical treatment

This includes processes in which permanent forming of the metal occurs using mechanical energy. These processes do not have an effect on the crystalline structure, but instead change the elasticity, tenacity, plasticity and HARDNESS.

The most common mechanical processes are:

25

In general, thermal treatment is simply heating and maintaining a certain temperature in the steel for a more or less long period of time. It should then be cooled in a suitable way. This results in changes to the microscopic structure of the steels that are responsible for the required adjustment of HARDNESS and STRENGTH.

This means that, in thermal treatment, only the thermal energy has an effect. This effect is on the crystalline structure, but not on the chemical composition.

The four most important thermal treatment methods are described in the following section:

Thermal treatment

Thermal treatment includes processes in which the properties of the metals are modified by changing their structure and the structural constitution. The resulting material with these new properties can perform the intended tasks better.

The application possibilities and performance are much better here than would be the case in normal conditions. Thermal treatment includes the thermal treatment of steel.

Process Procedure

Hardening The steel is hardened by cooling it quickly to avoid renewed changes to the crystalline structure.

Annealing The steel is annealed when it has been subjected to deformations through mechanical influences or other causes. The steel is cooled very slowly in this case.

Normalising The steel is normalised to remove tension in the steel. The normal state of the steel concerned is produced. A medium cooling speed is required for this.

Tempering The steel is heated (tempered) again after hardening. The stability of the structure is improved in the process. Tempering partly reverses the hardening. However, these effects become greater as the temperature to which the material is heated rises.

26

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Microstructures

Thermal treatment methods allow variations in the structural composition of the steel to be achieved. This is, of course, also dependent on the composition of the steel (proportion of alloying elements).

The respective variation is obtained using the proportions of certain structural components that have a basic influence on the STRENGTH/HARDNESS of a steel type.

Basics - Materials

The steel can be adapted to the application requirements with this kind of treatment.

This approach is used with the body. Depending on the requirements for the different body areas, steels with the necessary structural compositions and thus also the necessary strength parameters are used.

Elasticity limit Re of selected structural components

The basic structural components with their respective strength areas are shown in the diagram.

Bainite: 15220MPa

Martensite: 170250MPa

Pearlite: 5570MPa

Austenite: 88105MPa

Cementite: 215MPa

Ferrite: 28MPa

Elasticity limit Re

27

Thermochemical treatment

This includes processes in which chemical components are involved in the steel treatment in addition to thermal energy (heat, maintain temperature, cool). The microstructure and the chemical composition of the steel vary accordingly.

The most important thermochemical treatment methods are:

Process Procedure

Case hardening In case hardening, the surface of a steel part is enriched with carbon (carburization) creating the conditions for later hardening.

The part is heated and kept at a suitable temperature while simultaneously being in contact with materials that can release carbon. The enrichment with carbon can be achieved, for example, by means of gas carburizing, powder carburizing and carburizing in salt bath.

During the subsequent cooling, the layers produced are hardened while the non-carburized material core remains unchanged.

The process results in greater tenacity and impact strength.

Carbonitriding Carbonitriding is actually an advanced variant of case hardening. In this process, the surface of steel parts is also enriched with nitrogen in addition to carbon.

The enrichment is performed in a cyanide salt bath at corresponding temperatures. The material is then cooled in a controlled process. There is little warpage due to the low hardening temperatures and the milder quenchant.

In this process, hard outer layers with a low thickness are created in a simple and fast way.

Nitrating Nitration also forms hard layers on the surface of the steel part by forming nitrides.

These layers reach a high level of HARDNESS that is even greater than the HARDNESS of the layers formed in case hardening (see above).

This process is based on steel absorbing nitrogen in its microstructure.The process occurs at low temperatures which is why deformation is minimal. There is no quenching and no transformation of the structure.

28

Basics - Materials

Full aluminium bodies are not used for Volkswagen vehicles. Aluminium is used for individual selected components, for example:

Doors and lids on Phaeton Bonnet on Touareg

Weight is playing an increasingly important role in vehicle design. This results in particular from constantly rising targets for economic and environmentally friendly vehicles.

In addition to optimised solutions in body design, lower weight can be achieved by using lighter materials. Aluminium, for example, is being used more and more.

Production of aluminium

Aluminium is extracted from bauxite in a digestion process using caustic soda (NaOH). This method is called the Bayer process.

Bauxite:

Formed by the weathering of limestone and silicate stone in the appropriate climatic conditions

The name comes from its place of discovery, Les Baux (southern France).

Aluminium is readily found on Earth, but economic extraction has only been possible for around the past 100 years.

It is difficult to extract the aluminium from the ore as it is in a very stable oxide bond with oxygen. It cannot therefore be extracted from the ore by smelting with the aid of carbon as in iron production.

Not until the end of the 19th century was it possible to produce aluminium on a large scale using electrolysis with the dynamo invented by Werner von Siemens.

Basics - Aluminium

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Tita

nium

oxi

de

Silic

on o

xide

Iron

oxi

de

Aluminium oxide

Electrolysis

Molten aluminium

Primary aluminium pig

Bauxite

Bayer process

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Alloying aluminium

Aluminium alloys are used instead of pure aluminium to guarantee optimum material properties for the respective body components.

Aluminium is low STRENGTH in its pure state. The properties of aluminium can be changed by adding other elements, i.e. by ALLOYING. This concerns primarily the STRENGTH and the corrosion resistance. The most frequent alloy components are magnesium and silicon. These ALLOYS then form the basis for extruded profiles, cast nodes and aluminium sheet.

On an industrial level, the extraction of aluminium by electrolysis is performed in large iron vats of molten aluminium oxide that are lined with a coating of carbon. The vat serves as the cathode. Carbon blocks submerged in the aluminium oxide are used as anodes.

Carbon coating

Molten aluminium oxide

CathodeAluminium

Carbon block anodes

The aluminium is deposited on the floor of the vat since the distance to the floor is shorter than to the side walls. This causes a layer of liquid aluminium to form under the molten aluminium oxide. The aluminium is tapped off every 24 days and can be cast in bars.

The STRENGTH of aluminium alloys can be increased by forging and curing. One decisive requirement for the usability of aluminium in technical applications is its curing capability.

In curing, precipitation of brittle structure components within the alloy structure can be achieved with specific temperature control. This builds up inner resistance to deformation and results in an increase in STRENGTH.

30

Classification according to strength

Ultimate strengthin MPa

Classification according to steel family

Manufacturing method

Conventional steels Up to approx. 300 Deep-drawn steels Rolling

High-strength steels 300-480 Bake-hardening steel Bake hardening

350-730 Micro-alloyed steel;Isotropic steels

Grain refining and precipitation hardening

340-480 Phosphor-alloy steelInterstitial-free (IF) steel

Solid solution hardening

Higher-strength steels 500-600 Dual-phase (DP) steel Hard phases

600-800 Transformation induced plasticity (TRIP) steel

Hard phases

Ultra-high-strength steels >800 Complex-phase (CP) steel Hard phases

Ultra-high-strength thermoformed steels

>1,000 Martensite steel Hard phases

Steels for Body Construction

Classification of steels for body constructionDue to the large number of possible steel types, it is useful to classify them according to specific features. This allows steels to be classified according to their mechanical values, for example, ultimate strength and yield point. One example is the differentiation of conventional, high-strength, higher-strength and ultra-high-strength steels.

It is possible to classify steels according to their mechanical values, but this is imprecise. There is no fixed value for a boundary between the different steels. Normally quite a large bandwidth is involved. There can, for example, be cases where high-strength steels have identical or higher values than higher-strength steels. For this reason, steels are normally and more relevantly also grouped into families according to manufacturing method.

Steels from these families will be looked at over the following pages as they are increasingly used in body construction. In particular, the mechanisms for increasing the STRENGTH compared with conventional deep-drawn steels are explained. A balanced ratio between increasing strength and plasticity is decisive here.

Basically the STRENGTH of steels can be increased in four ways:

solid solution hardening grain refinement and precipitation hardening, bake hardening hardening using hard phases (hard phases = hard structure phases)

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Phosphor-alloy steels: These steels also have a ferrite matrix and contain hardening elements from solid solution hardening with phosphor (substitution), which can have a proportion of up to 0.12%. The steels are characterised by their compromise between mechanical STRENGTH and press formability, which is why they have a wide range of applications, for example, in structural or reinforcing parts (longitudinal members, crossmembers, pillars etc.), parts that need to be fatigue-resistant or parts that play an important role in crashes. These steels mostly have a minimum ultimate strength between 340 and 480 MPa or a minimum yield point between 220 and 360 MPa.

Increasing strength of steel

Increasing strength - with solid solution hardening

Normally, hardening initially calls to mind the thermal treatment of steel for achieving greater HARDNESS. Increasing the STRENGTH and also the HARDNESS can, however, be achieved by ALLOYING with other elements. This method is known as solid solution hardening. The hardening method allows the two steel types higher-strength IF steels (Interstitial Free steels = IF steels) and phosphor-alloy steels to be produced, for example.

Alloying material deposited interstitially (IF) (e.g. carbon and nitrogen)

ATOM of Fe basic lattice

Fe ATOM replaced with substitution (e.g. phosphor)

Higher-strength IF steels: These steels have a ferrite matrix without interstitially (interstitial = inside lattice) dissolved alloying proportions. As early as the steel melt, an accordingly lower proportion of carbon and nitrogen is set with specific treatment. Remaining ATOMS from these elements are bound by micro-alloying titanium and niobium. The steels have good malleability due to the lack of blockages by carbon and nitrogen (interstitial free). Phosphor and manganese are added to the steel to obtain the required STRENGTH. This is then known as higher-strength IF steel. The steels mostly have a minimum ultimate strength between 340 and 460 MPa or a minimum yield point (here the 0.2% offset yield point RP0.2) between 180 and 340 MPa.

The steels have a good balance between press formability and mechanical STRENGTH. If repairs are required, the steels weld well in any welding method.

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Increasing strength - by means of grain refinement/ precipitation hardening

The STRENGTH of a steel can also be increased by reducing the grain size (grain refinement) and with precipitation hardening. This principle of grain refinement is used, for example, for micro-alloyed steels. Vanadium, niobium and titanium are alloyed in small quantities. This micro-alloying creates a fine dispersion of carbide/nitride precipitation during hot-rolling. The grain growth is limited by this and ferrite grains measuring

33

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Bake hardening steels are easy to form in their original condition. They do not acquire their final higher strength until the formed component goes through the paint baking process. These steels meet the requirements for good formability and simultaneous component strength very well - this is a great advantage in the production of a body.

Raising the elasticity limit Re to Re(BH) is achieved with thermal treatment at a low temperature like paint baking in the paint oven. This is also known as a bake-hardening effect (BH effect) - accordingly heating and drying in the oven at 180C is compared with a baking process.

This heating process is sufficient to change the molecular structure of the sheet so that the elasticity limit is increased.

BH steels comprise a ferrite matrix, which contains the carbon required for the bake-hardening effect in the form of a solid solution. The improvement achieved through thermal treatment normally exceeds 40MPa. This means a steel with a fracture strength (Rp 0.2) of around 220MPa has a fracture strength of up to 260MPa following the bake-hardening effect. Bake-hardening steels mostly have a minimum ultimate strength between 300 and 480MPa or a minimum yield point of 180 to 360MPa.

Advantages of bake-hardening effect:

It improves the deformation resistance of all processed parts, even those with a slight deformation level (bonnets, roofs, doors, wings ).

Thinner sheets can be used to provide the same mechanical properties. This reduces the weight of the body and, at the same time, increases the deformation resistance.

BH steels are used for parts of the body panelling (doors, bonnets, tailgates, front wings and roofs) or for structurally important parts (pillar reinforcements or longitudinal members).

Due to the higher elasticity limit, more force is required for reshaping. It welds well with any method thanks to the minor ALLOYING.

Increasing strength - by means of bake hardeningElasticity limit (Re)after thermal treatment

Stre

ss (M

Pa)

Strain (%)

Original elasticity limit (Re)

Incr

ease

ac

hiev

ed w

ith B

H

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Increasing the STRENGTH of steels using hard phases leads to so-called multi-phase steels that achieve their STRENGTH through the coexistence of hard and soft phases - i.e. multiple phases - in their microstructure.

These hard and soft phases are structural components in the steel, which are characterised by their respective different HARDNESS.

Production of multi-phase steels

The production of multi-phase steel makes use of the property of steel to form different microstructures depending on the forming and cooling conditions.

These different structural components and, in particular, also intelligent combinations of them allow a very varied configuration of material properties adapted to customer requirements.

Today, multi-phase steels are produced with ultimate strengths ranging from 500 to around 1,400MPa.

The explanation of the multi-phase steels and their production - including simplified illustrations of the temperature and transformation processes - will use the treatment of hot-rolled metal strips (see page 36 37).

When a steel is manufactured with hard phases, the starting steel is subjected to a specific process and is thus transformed into a steel with modified structural composition.

Increasing strength - with hard phases

Different methods can be used for production depending on the manufacturer. For example, the required microstructure can be obtained later on by heating the cold-rolled metal strips.

Also the metal strips produced by hot-rolling can immediately be cooled accordingly and the required microstructure configured.

35

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The category of multi-phase steels includes

DP steels (dual-phase steels) TRIP steels (TRansformation Induced Plasticity steels) CP steels (complex phase steels) MS steels (martensite steels)

All of these steels are characterised by a high level of stiffness, high energy absorption and great deformation resistance. The most frequent applications are those in which a high level of energy absorption without deformation of the part is required, for example, in the reinforcement for the B-pillar or the inner reinforcements for the side members.

It is generally difficult to reshape this kind of sheet which is why the part is normally just replaced. However, weldability is excellent with any method.

Ferrite

Austenite

Martensite

Bainite

DP steels

Ultimate strength Rm

TRIP steels MS steels

CP steels

500600MPa

600800MPa

>800MPa

>1,000MPa

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TRIP steels have a very good ratio of STRENGTH and plasticity; the ultimate strength Rm is between 600 and 800MPa.

The STRENGTH is achieved by fast cooling of the hot metal strip - immediately after the rolling process. It is cooled in the ferrite area where ferrite is formed in a cooling pause. The austenite is enriched with carbon during this process. Then the bainite area is cooled and the hot metal strip is reeled up. The enrichment of the austenite with carbon continues. If the martensite starting temperature is below room temperature, the austenite that has not yet been transformed remains as so-called residual austenite in the structure. Undesired martensite forming in the bainite is prevented by suitable ALLOYING. The temperature of the reel is decisive for the process.

TRIP (TRansformation Induced Plasticity) steels

DP (dual-phase) steels

DP steels have a high mechanical STRENGTH and good malleability. The ultimate strength Rm is between 500 and 600MPa.

The STRENGTH is achieved by fast cooling of the hot metal strip - immediately after the rolling process in the ferrite area. A cooling pause in the ferrite area allows the formation of sufficient ferrite. Then it is cooled more quickly to a low reeling temperature so that the remaining austenite is turned into martensite.

The cooling speed and the steel composition are adjusted to each other so that no pearlite and as little bainite as possible are formed. A mixed structure results with approx. 8090% ferrite and 1020% insularly deposited martensite.

TRIP steels have up to 20% residual austenite in their structure. This residual austenite is not transformed into martensite until the material is shaped later on and is hardened in the process.

TRIP steels normally contain about 0.150.4% carbon, 12% silicon and 0.52% manganese.

Dual-phase steels contain approximately 0.12% carbon, 0.5% silicon and 1.46% manganese.

* Reel = reeling device for hot metal strip

Time

Tem

pera

ture Ferrite Pearlite

Bainite

Martensite

Time

Tem

pera


Bainite

Martensite

Rolling mill

Metal strip

Reel*

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CP (complex-phase) steels

CP steels mark the transition to the ultra-high-strength steels, which can have an ultimate strength value Rm of over 800MPa.

CP steels are characterised by high energy absorption and high deformation resistance.

The STRENGTH is achieved by fast cooling of the hot metal strip - immediately after the rolling process. It is cooled quickly to a reeling temperature in the bainite range and then reeled. Only a small amount of ferrite and martensite is formed. Micro-alloying additives like niobium and titanium are used for grain refining. This gives the steel a very fine-grained structure.

CP steels have a low carbon proportion of less than 0.2%; furthermore they contain alloying elements like manganese, silicon, molybdenum and boron.

MS (martensite) steels

MS steels are among the highest-strength steels. They are characterised by ultimate strength values Rm of around 1,000MPa to over 1,400MPa.

The STRENGTH is achieved by cooling the hot metal strip at maximum speed - immediately after the rolling process. In the process, it is cooled to approximately 200C and the strip is reeled. A predominantly martensitic microstructure is thus produced.

Due to the high STRENGTHS over 1,400MPa, thermoforming is necessary for the later shaping of parts made from martensitic steels.

As even small ferrite proportions need to be avoided due to the danger of material properties being dispersed, the transformation is controlled as required by alloying Mn, B and Cr.

The STRENGTH is controlled via the carbon content.

Time

Tem

pera


Bainite

Martensite

Time

Tem

pera


Bainite

Martensite

38


Body structureOverall, the body structure is the most important and central part of the body.

Today, modern bodies need to meet extensive and very complex requirements. They are therefore optimised according to the following main points:

Passive safety Lightweight construction Stability and vibration resistance Pedestrian protection Corrosion protection

In addition to the geometric/constructive design of the SEMI-FINISHED PRODUCTS and profiles, this optimisation is achieved by using tailor-made materials with different strengths for the respective body areas.

Steel is used more than any other material. Aluminium and plastics are also used to create lightweight constructions.

Sheets with higher STRENGTHS are increasingly being used the greatest STRENGTH is provided by thermoformed ultra-high-strength sheets.

What characterises these sheets?

They are steel sheets that are formed in heated state, for example, at temperatures between 900 and 950C. A defined cooling process produces a microstructure that provides greater STRENGTH and HARDNESS. There are also die-quenched sheets.

These allow, for example, very slim and light body parts to be produced without losing strength.

Ultra-high-strength sheets in the assemblies:

Inner A-pillar Seat brackets Outer side member Front brackets on front longitudinal

members Rear bumper cross member

Taking the latest Golf as an example, we will show the different materials that are used in the body assemblies

39

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Legend

Steel sheet up to 140MPa

High-strength steel sheet from 180 to 240MPa

Higher-strength steel sheet from 260 to 320MPa

Ultra-high-strength steel sheet from 340 to 700MPa

Ultra-high-strength, thermoformed steel sheet over 1,000MPa

Thermoformed ultra-high-strength sheets in the assemblies:

Front bumper cross member Cross member in footwell area Inner side members Centre tunnel A-pillar/roof frame area B-pillar

The strength figures in MPa refer to Rm = ultimate strength.

1 MPa = 1 newton/mm2

40

Basics - Process Engineering

Manufacture of semi-finished productsConstantly greater and also more differentiated constructive requirements are being set for modern bodies. To meet these requirements, the materials/SEMI-FINISHED PRODUCTS are selected and used so they are precisely tailor-made for the load profile.

The body calculations and later tests provide load curves within the body that can be predicted increasingly closer to practice thanks to todays computing possibilities.

The calculations and tests are used as a basis for the configuration of the body structure:

Defining the semi-finished product profile Selecting the semi-finished product material Selection of the respective body parts and their material that will be treated during

production (e.g. hot forming in forming die) Selection of the body areas and the respective parts that have cross-section variations

within the part (tailored blanks)

Design and calculation Laboratory and practical tests

41

The main forming methods are:

Deep drawing

Hydroforming

Impact extrusion

Rolling

Extrusion

Forming process

Different processes are used to produce SEMI-FINISHED PRODUCTS.

Todays thin steel sheets are ideal for meeting the requirements of lightweight construction while simultaneously providing high STRENGTH to meet safety requirements. However, they also need accordingly good malleability for shaping.

This shows how complex the requirements are!

42

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The cut sheet metal is placed on the drawing die

and clamped by the holder.

The punch is then pushed downwards and draws the sheet metal over the drawing edge into the opening in the drawing die. The sheet metal is pressed so firmly onto the drawing die by the holder that creases cannot form in the flange area.

Upon completion of the drawing process, the punch releases the workpiece as it moves upwards.

Deep drawing

In deep drawing, the sheet metal is formed by using the simultaneous effect of pulling and pushing forces. The sheet metal is shaped in one or more operations. Materials with corresponding deformability are used - deep-drawn sheet. One special type of deep-drawing is stamping. Embossed structures and shapes, for example, inscriptions, can be added to the surface - this is often performed simultaneously with the deep-drawing process, however.

Punch

BlankDrawing die

Forming force

Holder

Flange

Drawing edge

Pull-off edge

43

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The sheet-metal profile is placed in the lower half of a two-piece die.

The die is then sealed and filled with a liquid pressure medium.

The liquid pressure medium is then pressurized to approx. 170MPa. The resulting forces press the inserted sheet-metal profile against the die and thus produce the required new form.

Upon completion of the forming process, the mould halves separate and the finished formed part can be removed.

Hydroforming

Hydroforming uses pressure to shape metal. In this process, high pressure is used to press a sheet-metal profile against a hollow mould. This method also allows parts with different and complex geometries, e.g. with subsections, to be produced very quickly. Furthermore it allows weld seams to be avoided. The strain hardening that occurs during forming increases the component strength or stiffness.

Counterholder

Tube

Mould half

Mould half

Sealing die

Pressure medium

Subsection

Finished T piece

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Impact extrusion

Impact extrusion uses pressure to shape metal. The materials used in this process need to have a high level of malleability.

In impact extrusion, materials are pressed into solid or hollow parts using a punch and a die.

A slug is placed in the forming die as the starting material. When the punch pushes this material together under high pressure, it is forced on the other side into the die opening.

Forming hardens the material. Furthermore a corresponding surface quality can be achieved with the die.

Remover

Slug

Punch

Die

Starting shape (slug)

Finished part

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Extrusion

Extrusion uses pressure to shape metal.

In this process, heated materials are pressed through a die and extruded as a billet.

Different profiles can be produced depending on the shape of die used.

The material is compressed by the reshaping in the die and thus hardened accordingly.

Furthermore the desired surface qualities can be produced.

Rolling

Rolling uses pressure to shape metal.

Profiles, sheets, tubes and wires are fed between rotating rollers and shaped in the process.

The material is compressed by the rollers and thus hardened accordingly.

Furthermore the desired surface qualities can be produced.

DiePunch

Die holderBlock

Billet

Roller

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Tailored blanksAs the term tailored indicates, tailored blanks are parts that are made to measure. Blanks are the starting material.

Tailored blanks were originally developed to produce sheet metal plates with widths that could not be produced with the rolling technology of the time. Today, producers make use of the advantage that tailored blanks allow different steel grades and sheet thicknesses to be joined together in a finished part.

Example - floor pan - frontwith tailored blanks

Tailored blanks allow an increasingly better and more precise adjustment of the body structure to the loading of certain body sections. This allows parts with complex shapes and precisely fitting designs to be produced.

The individual parts of the tailored blanks are joined together by welding. After welding the blank is then shaped, for example, by deep drawing.

Increasing sheet thickness

Front longitudinal member

Front floor

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Different welding methods are used to join the individual components of tailored blanks.

One method that was frequently used in the past was mash seam welding. Today, this has, however, mostly been replaced by more modern laser welding. Therefore we will not look further into mash seam welding.

The seam edges must be prepared well to obtain the necessary seam quality in laser welding.

In addition to laser welding, high-frequency welding is also used for tailored blanks today.

In this process, metal sheets are clamped at a defined gap x from each other.

The high-frequency current is conducted directly through the sheets via clamping jaws/electrodes. Electrodynamic effects cause the welding current to be concentrated on the edges of the sheets. Once the necessary temperature has been reached, the current is switched off and the heated joining edges are squeezed together.

Extremely short welding times can be achieved with high-frequency welding. The requirements for the seam preparation are fewer than in laser welding. Even non-straight seams can be welded easily. One disadvantage of the process is the protruding seam left upon completion of the welding process. The seam needs to be reworked afterwards.

Welding rod feed

Welding rod

... joining with laser welding

Laser beam

Laser nozzle

Sheet metal

... joining with high-frequency welding

Clamping jaws/electrodes

Sheet metal

Protruding seam

HF connection

Sheet metal

48

The following joining methods are used for body construction:

Frictional connection:

Screw joint

Frictional connections, for example, screw joints, represent only a small proportion of the joining methods in body construction today. The necessary requirements for stability and stiffness of the body cannot be met with screw joints.

Positive-fit connection:

Rivet joint

Rivet joints are among the positive-fit connections. At least two parts are interlocked and connect the parts. Rivet joints also only meet the requirements for stability and stiffness to a limited extent.

Firmly bonded connection:

Weld joint Solder joint Adhesive joint

The aforementioned requirements can be easily fulfilled with firmly bonded connections. These joining methods produce a material compound since material is mixed in the join gap, e.g. in welding. Parts joined in this way can be considered to be more or less a new part. Material mixing does not occur during soldering and bonding. However, direct contact in the join gap is considerably more intensive so accordingly high STRENGTHS can be achieved.

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Joining processesMonocoque bodies need a structure that provides the necessary stability and stiffness - without a supporting frame. The joining methods used to produce the body are very important. Parts are connected to each other in joining.

49

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Screw joint

Screw joints have the advantage that repairs and replacement operations are easy and cheap to perform.

However, the disadvantage is that the body stiffness normally required for todays monocoque bodies is very difficult to attain.

In many cases, the advantages are minor compared with the advantages of a firmly bonded connection. A very stiff body is important with regard to crash safety in particular and also in general driving.

Screw joints reach their limits here.

For this reason, screw joints are used, above all, in places where the load bearing capacity of the body structure is not an issue.

Furthermore attachments are mounted with screws to a great extent.

Where are screw joints used, for example?

Fastening wings to the body structure Securing the front cross member to the

longitudinal members

Front left wing(Golf 2004)

Front cross member(Passat 2006)

Screws for front cross member on Passat 2006 (example)

Screws for front left wing on Golf 2004 (example)

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Rivet joint

Punch riveting

Punch riveting involves a semi-hollow rivet being pressed through the first layer of sheet metal. The second layer of sheet metal is only deformed by the rivet and is not penetrated. The foot of the semi-hollow rivet spreads and forms a closing head that gives the rivet joint the necessary hold.

Advantages:

No pre-drilling of sheets Second layer of sheet metal is not cut Greater STRENGTH and lower energy requirement

than with resistance spot welding

Steel is used as a material for punch riveting. If the rivets are used for aluminium sheets, they need to have a zinc/nickel coating to avoid galvanic corrosion.

Clinching

Clinching is used to connect non-load bearing simple components as the connection point only has a low STRENGTH. Rivets are not required here. The punch presses the two sheets into a die at the joining point. By sinking and clinching the upper sheet into the lower sheet, a frictional and positive-fit connection is formed.

Advantages:

Fast and clean joining technique Low cost

If the sheets are made from different materials, an electrochemical insulation coating is required to prevent galvanic corrosion.

Riveting tool (upper part)

Riveting tool (lower part)

Semi-hollow rivet

1st and 2nd sheet layers

Punch

Upper and lower sheet

Die

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Blind riveting

Blind riveting can be used whenever the riveting point can only be accessed from one side.

Pop rivets are hollow rivets with a mandrel.

The pop rivet is inserted into the aligned holes of the parts being connected with a rivet gun.

The gun now pulls the mandrel back. This causes the mandrel head to expand the protruding rivet body and form the closing head.

The mandrel then snaps at its break point.

The rivet gun is removed with the broken-off mandrel. The rivet joint is complete.

Rivet gun

Mandrel

Pop rivet

The illustrations show an example of blind riveting using pop rivets.

Parts being connected

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Examples of laser weld seams on the Passat 2006:

in the area of the door apertures on the front and rear roof member on the lateral windscreen frame in the area of the front wheel housing in the area of the rear window frame on the rear cross panel

In laser welding, the materials to be joined and the welding rod are liquefied by the high energy of the laser beam.

At high working speeds, a very good surface quality is obtained.

The surface of a laser weld seam is already so good that reworking, for example, for a good paint finish, only requires a minimum amount of time.

Laser welding allows an almost seamless connection of parts - this is particularly advantageous for connecting higher-strength sheets. The structural behaviour of the material is also largely retained in the area of the seam.

Complicated seams, for example, single-sided welds, are possible. In addition to butt welds, the use of an additional rod also allows other types of seam, for example, hollow welds.

Welding

Today laser welding and electric resistance welding are used for a wide range of welding applications. Therefore both methods will be looked at as examples of welding.

In addition to laser welding, plasma welding - a further development of TIG welding (TIG = tungsten inert gas) - is also used. This method is currently not very widespread in body construction and will therefore not be looked at here.

Laser welding

In laser welding, the heat to melt the materials is produced by a laser beam. Laser welding was first used to produce tailored blanks for body construction. It is finding increasing use in the assembly of vehicles thanks to many advantages:

Welding rod feed

Welding rod

Laser beam

Laser nozzle

Sheet metal

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Resistance spot welding:

Resistance spot welding continues to be very important in body construction. Today, new modern possibilities in control and regulation allow resistance spot welding to be also performed on higher-strength and coated sheet metals.

In modern body construction, adhesive is also added to the joining level to improve the structural behaviour and stiffness of the join and increase the vibration resistance. The adhesive layer also prevents penetration of substances, for example, water or other liquids and thus also helps prevent crevice corrosion. One variant of resistance spot welding is resistance roller spot welding that uses rollers as electrodes. The parts being joined run between the two electrodes and a weld seam is produced.

Electric resistance welding

This method allows electrically conductive materials to be welded. An electrical current flowing through the welding joint melts the material and the parts are welded together - without additional material. Normally low voltages and high currents are used so that the welding effect is possible, but there is also no danger.

This welding principle is applied in different ways, for example:

Resistance spot welding Resistance butt welding

Resistance butt welding

Resistance butt welding is only used on joints that do not carry any loads. Two parts are pressed together and connected to a current. The material melts and is welded at the pressure point. In body construction, this is used, for example, to weld bolts to sheet metal parts.

Weld

Electrode

Pin

Welding spots

Electrode

Sheet metal

Sheet metal

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Soldering

In contrast to welding, the actual parts being joined are not melted at the joint in soldering. An additional material made from bronze, the solder, is used. The melted solder fills the space at the joint and connects the parts being joined with a high STRENGTH. Due to the low temperatures used for soldering, the zinc coating on galvanised sheet metal, for example, is affected to a lesser extent and substantial hardening is avoided in the join area. Join gaps can be bridged better by the solder. The lower process temperatures allow less heat distortion.

The main soldering methods are explained briefly below.

TIG solderingMIG soldering

The solder is melted by the arc between the solder and the part being soldered under the inert gas.

The solder is melted by the arc between a tungsten electrode and the part being soldered under the inert gas.

Plasma soldering LASER soldering

Compared with TIG soldering, the arc is additionally constricted by a plasma nozzle in plasma soldering. This allows a higher energy density and narrower seams. Also higher soldering speeds are possible.

In LASER soldering, the soldering heat is produced by a laser beam.

Soldering seam

Solder (wire)

Wire feedInert gas nozzle

Arc

Sheet metal

Wire feed

Clamping sleeve

Solder (wire)

Tungsten electrode

Inert gas nozzle

Sheet metal

Soldering seam

Arc

Plasma nozzle

Wire nozzle

Solder (wire)

Wire feed

Tungsten electrode

Inert gas nozzle

Sheet metal

Soldering seam

Arc

Solder (wire)

Wire feed

Laser beamLaser nozzle

Sheet metal

Soldering seam

Wire nozzle

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according to processing temperature

Cold adhesives (room temperature)

Hot adhesives (120250C)

according to composition

Single-component adhesives

Two-component adhesives

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Bonding is not just used to join parts together, however. In body construction, in particular, the method is used for a range of other tasks, for example:

Sealing the joint area against penetrating moisture or corrosive substances

Joining different materials, for example, steel sheet with plastics

Noise and vibration insulation Gas and liquid sealing Increasing component stiffness Avoiding crevice and galvanic corrosion Low-warpage joins by avoiding temperature

loading Cross-area application of force

Appropriate preparations are necessary for proper bonding, for example:

Surfaces should be clean and free of grease A primer should be applied (bonding agent)

Bonding

Bonding is a method for the permanent connection of components using an adhesive. Depending on the type of adhesive, the hardening process can be supported by heating the bond slightly. Thanks to the continued development of new, more efficient adhesives that are tailored to the applications, adhesives and bonding processes are becoming increasingly important in body construction. Bonded joints can even be STRONGER than welding points, for example.

Adhesive

Different adhesives are used depending on the application.

They differ

Body part

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The following are normal zinc coating methods for bodies:

Hot dip galvanisingfor

all parts that are not visible from the outside

Galvanealed and zinc electroplatingfor

all outer skin panels

Rough surface Smooth surface

Zink coating approx. 10 micrometres Zink coating approx. 8 micrometres

Poor formability Good formability


Coating processesThe aim of the different surface coatings is to protect the steel in the body against possible oxidation (CORROSION) due to the effect of air and weather. Different coatings are used, but coatings based on zinc are most common.

In production today, the body is not completely galvanised by immersion, for example; the body components or their initial SEMI-FINISHED PRODUCTS will in fact be coated before further processing.

Zinc is a material that, compared with steel, tends to oxidise more greatly. This results from its position within the electrochemical series - according to the table, zinc is less precious than steel.

The zinc coating covers the steel surface and thus protects it against CORROSION. Optimum protection is provided as long as this coating system is not damaged - for example, by mechanical intrusion.

If, however, there is an intrusion which damages the zinc coating and reveals the surface of the steel, contact with water, for example, can lead to premature disintegration of the zinc coating due to the potential difference between the two different materials. The surface of the steel is then exposed and CORROSION starts on the respective steel surface areas.

Optimum protection is achieved if the zinc coating is combined with a coat of paint. This is known as a duplex system.

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Hot dip galvanising

Hot dip galvanising is performed after rolling and before deep drawing of the sheet metal. After rolling, the sheet metal is immersed in a 450C liquid zinc bath causing a coating of zinc to be formed. The first coatings of zinc consist of an iron/zinc compound and are followed by a coating of pure zinc. Hot dip galvanising is one of the most effective corrosion protection methods.

Production line for deep drawing

Zinc bath

Heating for zinc bath

Coiled galvanised metalstrip

Hot dip galvanising cannot be used for outer body panels due to the high process temperatures and the risk of the metal becoming warped.

Hot dip galvanising is not normally used on visible parts as it leaves a rough surface with markings on the sheet metal. Visible parts would need to be further processed.

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Galvanealed method

The coating formed in galvanealing is added after rolling and before deep drawing of the sheet metal.

The galvanealed method is a variant of hot dip galvanising. In contrast to hot dip galvanising, the steel is treated thermally (annealed) at 430C for an additional 30 seconds after the bath.

In this thermal treatment, the iron present in the steel is diffused into the zinc coating. An ALLOY of zinc and iron is formed. The zinc coating contains around 10% iron and is known as a galvanealed coating.


Zinc bath

Galvanised metal stripcoiled

Thermal treatment

Heating for zinc bath

Galvanealing provides greater corrosion resistance compared with hot dip galvanising

This method has the following purpose among others:

Optimisation of coating weldability Good surface quality without irregularities Good basis for subsequent coatings.

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Zinc electroplating

Zinc electroplating can be used for outer body panels due to the low warpage.

Zinc electroplating is performed after rolling and before deep drawing of the sheet metal. This method is based on an electrochemical process.

The metal strip is guided over so-called current transfer rollers, which act as the negative pole (cathode). An electrical field between the metal strip (which now also acts as the cathode) and the anodes acting as the positive pole causes zinc released from the electrolyte liquid to be deposited on the sheet.


Galvanised metal stripcoiled

Anode

Cathode(current transfer rollers)

Electrolyte liquid

This creates a very fine, uniform zinc coating with a thickness of about 8 micrometres. Subsequent processes, like stamping, welding and painting, can then be carried out without losing the corrosion protection.

Due to the good final quality, this technique is used for visible parts of the body.

The final appearance of the zinc electroplating depends on the coating thickness and the surface quality of the steel.

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Corrosion Protection

Pre-treatmentIf the body is to receive a coat of paint, the material must be prepared well. Phosphating is mainly used in body construction for this purpose.

Aims of pre-treatment:

Corrosion protection Creation of a good primed surface

These phosphates are produced in a chemical reaction with the base metal and thus form a layer that is securely anchored to the base metal. Depending on the reacting base metal (this can also be the coating material), a layer of, for example, iron, nickel, manganese or zinc phosphate is formed.

Phosphating

Phosphating is chemical/electrochemical process in which thin fine crystalline water-insoluble phosphates are deposited on the surface of the metal by dipping it into phosphoric acid solutions (iron, zinc or manganese ions and phosphoric acid).

The coating has numerous hollow spaces and capillaries, which are linked to optimum absorption. This allows good penetration of corrosion protection agents like waxes, oils, colour pigments and paints.

Phosphated surfaces thus ensure a good primed surface for paint and lacquer coatings.

Conveyor

Phosphating bath

Phosphate

Body

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Seam sealing

Welding spots (spot welding)

Adhesive

Weld seam with seam sealing

Seam sealingEffective seam sealing is equally important for permanent corrosion protection.

It is often presumed that, in particular, missed spots in the body cavity sealing are the starting points for CORROSION - today the process of body cavity sealing has been perfected so that hardly any problems can occur in this area.

The situation with seam sealing is more critical, however.

At weld seams with overlaps, for example, on

front and rear skirts rear valance engine compartment luggage compartment floor passenger compartment floor side panel wheel housing vehicle underbody wings etc.

the seams require sealing afterwards with suitable sealing materials so that it is impossible for water to get into the spaces between two joined components.

Even if, for example, the gaps of weld seams (spot welding) are filled with adhesive, the seams must be sealed. Seam sealing also protects the edges of the sheets.

Sealing seams is important because it is virtually impossible for any moisture that penetrates the seams to dry. CORROSION can start in seams considerably faster compared with open surfaces.

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Corrosion Protection

Applying a protective sealant after priming (dip painting) and before applying the paint e.g. at front end of body.

Stone chip protectionStone chip protection is one important and, in practice, also very effective measure against CORROSION. The complete body cannot be treated in this way. Instead coats are applied to the areas of the body that are at risk. This can be done in the following way:

Stone chip protection is normally a tougher, elastic coating. The elastic properties combined with a greater thickness also have an anti-drum effect.

Stone chip protection can also be achieved by using special materials for body parts, for example, by using special plastics instead of painted metal parts.

Applying protective sealant after painting (clear coat) to non-visible areas, e.g. to the vehicle floor and the wheel housings, or also applying a special protective film.

Body material

Phosphate

Dip painting

Filler coat

Base coat

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